sub BackAndForth {     
     my ($filename) = @_;


      #  make multi-dimensional hash here ... it looks like
      #  $ALLDATA{$filename}{$key1}{$key2}{$key3} = [@array]
      
      return (%ALLDATA);     
}

      my %ExternalHash = &BackAndForth('filename.csv');
         %ExternalHash = &BackAndForth('filename2.csv');

##</code><code>##

          %ALLDATA = (
               'key1' => HASH(0x9021f6c)
                   'key2' => HASH(0x90154a8)
                       'key3' => ARRAY(0x9015400)
                                  0  value1
                                  1  value2


##</code><code>##

   %thisDataset = (
          '' => HASH(0x876f324)
                'key2' => HASH(0x9029220)
                       empty hash)

##</code><code>##

       return(\%ALLDATA)
       %external = (yadda yadda)
             or 
       $external = (blah ...